There have so far been developed and manufactured optical pickup devices (which are also called optical heads or optical head devices) for conducting reproducing/recording of information for optical information recording media (which are also called optical discs or media) such as CD (compact disc) and DVD (digital video disc, or digital versatile disc), and they have generally been popularized.
Further, standards for an optical information recording medium which allows higher density information recording have also been studied and developed.
In the optical pickup device of this kind, a light flux emitted from a light source (a laser diode is mainly used) is converged on an information recording surface of an optical disc through an optical system composed of a beam forming prism, a collimator, a beam splitter and an optical element such as an objective optical element, to form a spot, and a reflected light coming from an information recording pit (which is also called a pit) on the recording surface is converged on a sensor this time through the optical system again, so that information may be reproduced through conversion to electric signals. In this case, a light flux of the reflected light is changed depending on a shape of an information recording pit, which is used to distinguish information “0” from information “1”. Incidentally, a protective substrate (a protective layer made of plastic which is also called a cover glass) is provided on the information recording surface of the optical disc.
When recording information on a recording type medium such as CD-R or CD-RW, a spot by a laser light flux is formed on a recording surface to generate thermo-chemical changes on a recording material on the recording surface. Due to this, irreversible changes take place on dyes, in the case of CD-R for example, and a form which is the same as an information recording pit is formed. In the case of CD-RW, a material of a phase-change type is used, and therefore, thermo-chemical changes cause reversible changes between the crystalline state and the amorphous state, which makes it possible to rewrite information.
In the optical pickup device for reproducing information from an optical disc that meets CD standards, NA of an objective lens is about 0.45 and a wavelength of a light source to be used is about 785 nm. For recording use, an objective lens whose NA is about 0.50 is commonly used. Incidentally, a thickness of a protective substrate of the optical disc meeting CD standards is 1.2 mm.
Though CD has been popularized widely as an optical information recording medium, DVD has been spreading during the past few years. The DVD is one wherein a storage capacity for information to be recorded has been increased by making a thickness of the protective substrate to be thinner than that of CD and by making an information recording pit to be smaller, and it has a huge storage capacity which is as high as about 4.7 GB (gigabyte) for about 600-700 MB (megabyte) of CD, to be frequently used as a distributed medium in which an animated image such as a movie is recorded.
Further, an optical pickup device for reproducing information from an optical disc meeting DVD standards is the same as that for CD in terms of principle. However, an optical pickup device wherein NA of an objective lens is about 0.60 and a wavelength of a light source to be used is about 655 nm is used as the optical pickup device for reproducing information from an optical disc meeting DVD standards, because information recording pits are made smaller as-stated above. For recording use, an objective lens whose NA is about 0.65 is commonly used. Incidentally, a thickness of a protective substrate of the optical disc meeting DVD standards is 0.6 mm.
With respect to an optical disc meeting DVD standards, those of a recording type are already put to practical use, and there are various standards including DVD-RAM, DVD-RW/R and DVD+RW/R. The technical principle for these is also the same as that of CD standards.
As stated above, further, the optical disc with higher density and higher capacity is now being proposed.
This optical disc is one that mainly employs a light source having a wavelength of about 405 nm which is one using the so-called blue-violet laser light source.
With regard to “the high density optical disc” of this kind, even when a wavelength to be used is determined, a protective substrate thickness, a storage capacity and NA are not determined equally.
Selection of the direction to improve recording density sharply makes a protective substrate thickness of an optical disc to be thin, and thereby, makes NA to be great. On the contrary, it is possible to make the protective substrate thickness and NA to be the same as standards of conventional discs such as DVD. In this case, the performance required as an optical system is relatively eased, although physical recording density is not raised sharply.
Specifically, there are proposed a thickness of a protective substrate that has been thinned further to 0.1 mm and a thickness of a protective substrate that has been made 0.6 mm to be the same as that of DVD.
Many of optical elements used in the optical pickup devices stated above are mostly those which are made of plastic resin through injection molding, or those which are made of glass through pressure molding.
Among them, the latter one representing an optical element made of glass has a refractive index which is varied less by temperature changes in general, and is used for a beam forming prism arranged near the light source representing a heat source. However, it has a problem that its manufacturing cost is relatively high. Therefore, its application to various optical elements such as a collimator, a coupling lens and an objective optical element has been reduced. In contrast to this, the former one representing an optical element made of plastic resin can be manufactured at low cost, which is a merit, and is used widely in recent years. However, the refractive index of the plastic material is sharply lowered by the temperature changes, resulting in a problem that the optical performance is seriously changed. This rate of the change by temperature is sometimes about 10 times the rate of the change in glass material.
Incidentally, for reproducing (reading) information at high speed or for recording information at high speed, it is necessary to increase an amount of light for forming a light-converged spot securely.
The most simple way for that purpose is to raise an amount of light emitted by a diode by increasing power of a laser diode, in which, however, heat is generated and working temperature also rises sharply. Thus, the refractive index of the plastic material is lowered as stated above, resulting in a problem that the designed optical performance cannot be attained.
Further, in the operations at high speed, an actuator works at high speed, and thereby, heat is further generated, and changes in refractive index of the optical element are caused, which is a problem.
Therefore, there are proposed various technologies to restrain changes in optical performance of the plastic optical element for temperature changes.
In Japanese Un-examined Patent Application Publication No. H11-337818, there is described an objective lens for an optical head wherein changes in wavefront aberration caused by temperature changes can be controlled to be small, and a range of usable temperature can be broadened even in the case of using as an objective lens with high NA. Specifically, the objective lens is a single lens made of resin whose both surfaces are convex and aspheric, and a diffractive lens structure representing a pattern of ring-shaped zones whose center is on the optical axis is formed on a lens surface on one side. The diffractive structure has a spherical aberration characteristic wherein spherical aberration changes in the direction toward insufficient correction when a wavelength of incident light is shifted to the longer wavelength side.
However, when providing the diffractive structure, there are various problems in the courses from processing of a die for injection molding to the actual injection molding, which causes a cost increase.
Further, when replacing an optical disc of a certain type with that of another type, the diffractive structure is used for correcting spherical aberration caused by a difference between substrate thickness or a difference between working wavelength, on one occasion, and the diffractive structure is used for correcting chromatic aberration caused by a change of a working wavelength resulting from temperature changes, on another occasion.
Under the aforesaid condition, it is not allowed to use the diffractive structure only for correcting temperatures, and the degree of freedom in design for each point is lowered. In the case of the temperature correction and correction of spherical aberration, in particular, the direction for correcting spherical aberration caused by changes for the former is opposite to that for the latter, which sometimes causes butting in design.